1. Field of the Invention
The present invention relates in general to a vaporizer apparatus for vaporizing a liquid material in order to provide a gaseous material for a chemical vapor deposition apparatus, for example. In particular, it relates to a vaporizer apparatus suitable for producing a vapor material for making a thin film having a high dielectric constant such as barium or strontium titanate.
2. Description of the Related Art
In recent years, there has been a remarkable advancement in circuit density of integrated circuit devices produced by the semiconductor industries, and intense development activities are in progress in anticipation of gega-bit order DRAMs replacing the prevailing mega-bit order DRAMs of today. Dielectric thin film materials used to make high capacitance devices necessary for DRAMs have, in the past, included silicon oxide or silicon nitride films of dielectric constant less than 10, or tantalum pentaoxide (Ta.sub.2 O.sub.5) films of dielectric constant of approximately 20. However, newer materials such as barium titanate (BaTiO.sub.3) or strontium titanate (SrTiO.sub.3) or mixtures of these compounds appear to be more promising.
In order to make such thin films, chemical vapor deposition (CVD) process is deemed to be promising. In this case, it is necessary that a feed gas must ultimately be supplied in a stable gas stream to a substrate disposed in a reaction vessel. In order to stabilize the vaporization characteristics, the feed gas is derived by heating a liquid source. The liquid source is produced by dissolving such materials as Ba(DPM).sub.2 or Sr(DPM).sub.2, which are solid at normal temperature, in some organic solvent (for example, THF).
A conventional vaporizer apparatus is disclosed in Japanese Patent Application Laid Open Publication No. H6-310444, for example, in which a liquid material or an atomized liquid material is injected into a chamber-type vaporizer which is heated by a heater. It also discloses a vaporizing plate in front of an injection nozzle for providing an improved heat conductivity for a better vaporization efficiency.
The vaporized gas prepared in the vaporizer is led into a reactor (film depositing chamber) through a feed gas distribution pipe, and is then led to the ejection head to be mixed with a reactive gas (oxidizing gas) supplied from the reactive gas delivery pipe. The vapor jet is directed to the substrate to produce a reaction so as to deposit a thin film (oxide film) on the substrate.
The feed gas used in such a deposition apparatus for making high dielectric films has the following intrinsic properties which present certain operational difficulties. They are: (1) the vaporization and decomposition temperatures are close together; (2) a difference exists in the vaporization temperatures between the film producing gas and the organic solvent; and (3) the vapor pressures are all very low. These characteristics cause difficulty in vaporizing the material and maintaining the stable flow of vaporized gas.
In the conventional technology, because the heat for vaporization is supplied by conduction from vaporizer plates disposed on the walls or interior of a box-shaped vaporizer, the vaporization efficiency from liquid to gas phase is not adequate. Also, the pressure within the reaction vessel is of the order of several torrs, for example 5 torr, resulting in a large difference in the pressures between the vaporizer and the reaction vessel. Therefore, as the gas passes through the vaporizer, the feed gas pressure is gradually reduced. The reduction in pressure is accompanied by volume expansion, so the velocity of the feed gas through the vaporizer is gradually accelerated. Consequently, there is a possibility that the feed gas passes through the vaporizer at high velocity without being completely vaporized.
Therefore, to prevent such a phenomenon and to vaporize the feed material completely within the vaporizer, it is necessary to provide sufficient dwell time for the feed gas to be vaporized. However, the size of the vaporizer necessary to accomplish such an objective becomes too large, and the length of the feed gas distribution pipe (from the vaporizer to the film depositing chamber) becomes too long. Thus, the possibility of an undesirable premature precipitation reaction from the gas phase increases. This further creates a cost problem for additional equipment and maintenance work for maintaining the feed gas distribution pipe at a high temperature.
Furthermore, if the feed gas and the reactive gas are delivered into the reaction vessel through separate delivery systems, the distributions of the feed gas and the reactive gas above the substrate surface can be non-uniform. This creates a product quality problem of composition or thickness non-uniformity in the film on the substrate which is a direct reflection of the non-uniformity of the feed gas.